This invention relates to the automatic control of electric energization applied to precipitators or to similar types of electrical apparatus subject to sparking.
Precipitator systems utilize adjacent electrodes having a large potential difference between them. Gases or fluids passing through these electrodes are exposed to an electrical field and are ionized such that undesired particles are attracted to the electrodes and thus, are removed from the gas or fluid stream.
The efficiency of particle removal is directly related to the magnitude of the voltage difference between the electrodes. However, an excessive potential results in sparking or in a more severe condition termed "arcing". Sparking, unless quickly inhibited produces arcing.
Arcing may, of course, also result from other causes, such as failures of the precipitator electrodes. During arcing, the precipitator system does not perform its precipitation function and additionally, consumes undesirable and large amounts of electrical energy.
Maximum efficiency of precipitator systems is believed to occur with maximum average DC electrode, i.e. ionization, current. If the electrode potential is substantially below the sparking level, the ionization current and, therefore, the precipitator efficiency is reduced. Conversely, if the electrode potential is too high, sparking and arcing results. Precipitator efficiency is similarly reduced with excessive sparking and with arcing. Precipitator systems should therefore be operated just at the sparking threshold. This maximizes efficiency and minimizes the production of destructive arcing.
Sparking is affected by many variable parameters, and therefore, may initiate at constantly varying magnitudes of electrode potentials and currents.
Accordingly, an adaptive spark testing process is utilized wherein electrode voltage is increased until sparking is detected and is subsequently reduced. Such precipitator control systems produce an increasing ramp signal which provides for the increase, with time, of the electrode voltage. Responsive to spark detection this ramp signal is reduced, i.e. set back, prior to resuming its increase. Such an adaptive type of sawtooth or ramping control causes the electrode potential to continuously increase to the sparking level, to be set back upon sparking, and to ramp upward again to the sparking level.
Numerous parameters affect successful and efficient operation of the system. For maximum efficiency, the electrode potential must be maintained for maximum time intervals closely adjacent to the sparking potential, and the rate and severity of sparking must be controlled.
It has been recognized that there are sparks of different severity. Some sparks of minimal intensity or duration may not produce arcing. However, remedial action must be taken in respect to other types of sparks. Additionally, maximum precipitator efficiency is attained if the number of potentially harmful sparks per unit of time is maintained at a low spark rate predetermined for the selected process and system. Spark rate is a function of the slope of the upward ramp, and of the magnitude of set back. Maximum efficiency is attained by a small set back and a small slope of the upward ramp. This provides for continuous excursion of the electrode potential close to the sparking level. Proper adjustment of the slope of the upward ramp and of set back also provides the desired spark rate.
However, it has also been found that upon detection of a potentially harmful spark, the electrode potential must be reduced sufficiently in magnitude and time duration to quench electrode ionization current. The actual turn-off requirements depend upon the precipitator system and the type of process. If there is insufficient turn-off, sparking is sustained and arcing is induced subsequent to set back. This objective could be met by setting back the ramp sufficiently to reduce the electrode voltage from a potential adjacent to the sparking level, e.g. 50 Kvs to zero and ramping upward to the sparking potential. This results in a drastic reduction of the ramp and a subsequent ramp slowly increasing from zero toward the sparking potential. At the required low spark rates, this results in a drastic reduction of average electron potential and current and a drastic reduction in efficiency. Accordingly, it has been found desirable upon spark detection to almost instantaneously reduce the electrode voltage to a minimum, e.g. zero volts. After a very brief turn-off time interval, the electrode potential, and thus the electrode current, is increased at a fairly rapid rate, the recovery rate, to a magnitude slightly below the sparking level. Subsequently, the electrode potential is again ramped upward at the previously described slow rate until sparking is again encountered.
The above described turn-off interval and recovery rate must be accurately selected for the particular precipitator system and process which is utilized. This is to assure that the ionization current in the precipitator is sufficiently quenched so that arcing does not resume, and that the subsequent turn on of the AC phase control system, e.g. solid state switching means such as back to back connected silicon controlled rectifiers, is not excessively fast. Conversely, a minimum permissible turn-off and maximum permissible rapid recovery rate improve the efficiency of the precipitation process.
Each of the above recited parameters can be controlled by adjustments within the control circuit. However, frequently there are undesirable interactions between them.